Encapsulation of electronic components with anisotropic thermoplastic polymers

ABSTRACT

An improved thermoplastic molding composition is provided which is particularly suited for use in the impervious void-free encapsulation on a relatively expeditious basis of an electronic component wherein the preformed electronic component is positioned in a mold cavity prior to the introduction of the molding composition via injection molding. The molding composition comprises a melt processable thermotropic liquid crystalline polymer which is of a relatively low weight average molecular weight of approximately 4,000 to 25,000 (e.g. approximately 4,000 to 10,000) and which is substantially incapable of further chain growth upon heating. Uniformly dispersed within the liquid crystalline polymer is approximately 40 to 80 percent by weight (e.g. approximately 50 to 75 percent by weight) of a particulate inorganic material (preferably of silicon dioxide) which serves to advantageously decrease its volumetric coefficient of thermal expansion and to advantageously increase its thermal conductivity. In a preferred embodiment the electronic component which is encapsulated is a semiconductor device, such as relatively delicate quad or dual-in-line integrated circuit device which is assembled onto a flat prestamped lead frame or other conductive device having a plurality of leads which extend outside the area which is encapsulated. The resulting encapsulated electronic component is well protected in spite of the relatively low molecular weight of the liquid crystalline polymer and is capable of satisfactory service for an extended period of time even if adverse environmental conditions are encountered. A rugged commonly non-burning and relatively inexpensive package is provided.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of our now abandoned U.S. Ser. No.517,870, filed July 27, 1983 entitled "Improvements in the Encapsulationof Electronic Components."

BACKGROUND OF THE INVENTION

Specific techniques for the encapsulation of electronic componentswithin a protective synthetic resinous material are well known in theart and are widely practiced. See, for instance, the article entitled"Encapsulation of Semiconductor Devices" appearing in Plastics DesignForum Issue, April, 1981, at Pages 49 to 54.

Heretofore, thermosetting resinous materials commonly have been employedto bring about the desired encapsulation through a form of injectionmolding commonly termed transfer molding. For instance, epoxy resins(e.g. novolac-hardened epoxy systems) commonly have been employed forthis purpose. Also, thermosetting resins such as unsaturated polyesters,bis-imido polymers, etc., have been proposed for use as encapsulationmaterials. See, for instance U.S. Pat. Nos. 4,327,369; 4,374,080; and4,390,596. Such thermosetting materials often require refrigerationprior to use, tend to require relatively long cycle times duringmolding, and after molding must be cured for extended periods of time atelevated temperatures in an oven before the desired cure of theencapsulant is complete. Since the viscosity of the uncuredthermosetting resin increases with curing as the resin is heated,relatively prompt use of the uncured material must be made once heatingis commenced. With such materials the recycle of scrap is impossible.Additionally, such materials having naturally occurring resin binderscommonly have a tendency to flash and to adhere to the surfaces of themold cavity causing possible mold damage thus requiring the substantialremedial attention of skilled personnel during the course of a moldingrun. This may preclude fully automating the encapsulation process.

While not utilized on a commercial scale, certain thermoplastic resinshave heretofore been proposed for use in the encapsulation of electroniccomponents. See, for instance, U.S. Pat. No. 4,327,369 at Col. 6, lines18 to 23, where passing reference is found to polyvinyl chloride,polyolefins, such as low-density polyethylene, high densitypolyethylene, polypropylene, and polystyrene. See also, U.S. Pat. No.4,370,292 where a polyphenylene sulfide composition which includes aphenoxy resin is proposed for encapsulation.

It is a common practice of the prior art to include within theencapsulating resin a particulate filler material such as silica oralumina which serves among other things to increase the thermalconductivity and to decrease the volumetric coefficient of thermalexpansion of the molded composition. Such particulate fillers, however,greatly modify (i.e. increase) the viscosity of the composition duringmolding especially when present in high concentrations. If the viscositybecomes too great the molding composition becomes difficult to cause toflow and to satisfactorily fill the mold. If voids are present in themolded article the encapsulation will be considered a failure. If theviscous composition is caused to flow through increased pressure, thismay damage the delicate electronic component undergoing encapsulation.This damage is termed "wire sweep" in U.S. Pat. No. 4,374,080. Suchsweep or deflection may severly stress or break the electrical circuitas bonds are torn or cause deleterious shorting. Also if one attempts toachieve the requisite viscosity for the molding composition through thereduction of the molecular weight of a thermoplastic encapsulant, thenthe resulting molded article commonly will possess inadequate mechanicalproperties (e.g. brittleness). If the polymeric material possessescontamination inherent in many polymerization processes (e.g.water-extractable halogens or water-extractable ionic materials), suchcontamination may attack the encapsulated electronic component and/oradversely influence its operation. Also, if the polymeric material has apropensity substantially to evolve gaseous by-products for any reason(e.g. because of a condensation polymerization reaction) during themolding operation or during subsequent use of the encapsulatedelectronic component, this can lead to excessive voids and the failureof the encapsulant composition. In addition to the above shortcomingsmany thermoplastics are incapable of prolonged reliable service at theelevated temperatures commonly encountered by electronic componentsand/or readily burn when subjected to flame thereby diminishing theirprotective properties.

It is an object of the present invention to provide an improved moldingcomposition which is particularly suited for use in the imperviousencapsulation of an electronic component.

It is an object of the present invention to provide an improvedthermoplastic molding composition which is particularly suited for theencapsulation of a delicate electronic component such as a quad or adual-in-line integrated circuit.

It is an object of the present invention to provide an improvedthermoplastic molding composition containing a substantial quantity of aparticulate inorganic material which is capable of encapsulating adelicate electronic component without damage to yield a finalencapsulated product which exhibits highly satisfactory mechanical,thermal, chemical, and electrical properties which render it capable ofsatisfactory service for an extended period of time even if adverseenvironmental conditions are encountered.

It is an object of the present invention to provide an improved methodfor encapsulating an electronic component which can be carried outwithout the need for a time consuming polymer curing step as commonlypracticed in the prior art.

It is another object of the present invention to provide an improvedencapsulated electronic component.

It is a further object of the present invention to provide an improvedencapsulated electronic component wherein the filled encapsulating resinis substantially impervious to water and ultraviolet light, issubstantially void-free, exhibits satisfactory mechanical properties(e.g. mechanical strength), exhibits a satisfactory volumetriccoefficient of thermal expansion, exhibits a satisfactory thermalconductivity, contains less than 50 parts per million ofwater-extractable alkali metal, contains less than 100 parts per millionof water-extractable halogen, exhibits a V-0 burning rating whensubjected to the UL-94 test, and is capable of extended service.

These and other objects as well as the scope, nature, and utilization ofthe claimed invention will be apparent to those skilled in the art fromthe following detailed description and appended claims.

SUMMARY OF THE INVENTION

It has been found that a molding composition which is particularlysuited for use in the impervious encapsulation of an electroniccomponent via injection molding comprises (a) a melt processable polymerwhich is capable of forming an anisotropic melt phase, has a weightaverage molecular weight of approximately 4,000 to 25,000, and which issubstantially incapable of further chain growth upon heating, and (b)approximately 40 to 80 percent by weight based upon the total weight ofthe molding composition of a particulate inorganic materialsubstantially uniformly dispersed in component (a) which is capable ofdecreasing the volumetric coefficient of thermal expansion andincreasing the thermal conductivity of component (a).

It has been found that an improved method for imperviously encapsulatingan electronic component comprises:

(1) introducing the electronic component to be encapsulated within amold cavity,

(2) completely filling the mold cavity surrounding the electroniccomponent by injection at an elevated temperature of a moldingcomposition comprising (a) a molten melt processable polymer which iscapable of forming an anisotropic melt phase, has a weight averagemolecular weight of approximately 4,000 to 25,000, and which issubstantially incapable of further chain growth upon heating, and (b)approximately 40 to 80 percent by weight based upon the total weight ofthe molding composition of a particulate inorganic materialsubstantially uniformly dispersed in component (a) which is capable ofdecreasing the volumetric coefficient of thermal expansion andincreasing the thermal conductivity of component (a),

(3) cooling the contents of the mold cavity to allow the moldingcomposition to solidify and to form an impervious package around theelectronic component, and

(4) removing the resulting encapsulated electronic component from themold cavity.

It has been found that an improved article of manufacture comprises anelectronic component impreviously encapsulated within a composition ofmatter comprising (a) a melt processable polymer which is capable offorming an anisotropic melt phase, has a weight average molecular weightof approximately 4,000 to 25,000, and which is substantially incapableof further chain growth upon heating, and (b) approximately 40 to 80percent by weight based upon the total weight of the composition ofmatter of a particulate inorganic material substantially uniformlydispersed in component (a) which is capable of decreasing the volumetriccoefficient of thermal expansion and increasing the thermal conductivityof component (a).

DESCRIPTION OF PREFERRED EMBODIMENTS

A wide variety of electronic components may be effectively encapsulatedwithin an impervious substantially void-free package in accordance withthe concept of the present invention. Such components may be relativelysimple electronic devices or relatively complex entities and may beregarded as elements for use in a larger electronic system.Representative components which may be encapsulated are transistors,capacitors, relays, diodes, resistors, networks of resistors, integratedcircuits, etc. In preferred embodiments the electronic components aredelicate semiconductor devices. These may be bipolar, field-effectdevices, etc. The integrated circuits which may be encapsulated can beprovided in a variety of configurations and the relatively small siliconor other semiconductor chips which commonly support the same may have asfew as 2 or up to 100, or more, contacts to the outside world. As iswell known in the art, typical integrated circuit packages commonly arefabricated on a thin metal frame of approximately 0.01 inch in thicknesswhich is composed of copper or of copper which has been plated withanother metal, such as a tin-alloy. A solder coated lead frame can beprovided. In such instances a subsequent soldering dip of the projectingelectrical contacts following encapsulation will not be necessary. Theintegrated circuit die or chip is cemented to the paddle portion of thelead frame, and the die or chip is electrically connected to the leadframe with thin wires commonly of gold having a diameter ofapproximately 0.001 inch or less. These wires are spot welded orotherwise attached from the contact or bonding pads on the integratedcircuit die or chip commonly formed of extremely thin aluminum films tothe ends of cantilever arms on the lead frame. Accordingly, in aparticularly preferred embodiment the electronic device is a quad or adual-in-line integrated circuit device which is assembled onto a flatprestamped lead frame having a plurality of leads which extend outsidethe area which is encapsulated. For instance, a 40 pin lead frame may beselected for encapsulation.

The first essential component of the molding composition in accordancewith the concept of the present invention is a melt processablethermotropic liquid crystalline polymer which has a relatively lowweight average molecular weight of approximately 4,000 to 25,000, andwhich is substantially incapable of further chain growth upon heating atits melt processing temperature.

As is known in polymer technology a thermotropic liquid crystallinepolymer exhibits optical anisotropy in the melt. The anisotropiccharacter of the polymer melt may be confirmed by conventional polarizedlight techniques whereby crossed-polarizers are utilized. Morespecifically, the anisotropic or ordered nature of the melt phase mayconveniently be confirmed by the use of a Leitz polarizing microscope ata magnification of 40X with the sample on a Leitz hot stage and under anitrogen atmosphere. The amount of light transmitted changes when thesample is forced to flow; however, the sample is optically anisotropiceven in the static state. On the contrary typical melt processablepolymers do not transmit light to any substantial degree when examinedunder quiescent conditions and are isotropic in nature.

Representative classes of polymers from which the thermotropic liquidcrystalline polymer suitable for use in the present invention may beselected include wholly aromatic polyesters, aromatic-aliphaticpolyesters, wholly aromatic poly(ester-amides), aromatic-aliphaticpoly(ester-amides), aromatic polyazomethines, aromaticpolyester-carbonates, and mixtures of the same. In preferred embodimentsthe thermotropic liquid crystalline polymer is a wholly aromaticpolyester, or a wholly aromatic poly(ester-amide). A polymer isconsidered to be wholly aromatic when each moiety present within thepolymer chain contributes at least one aromatic ring. Also, it ispreferred that naphthalene moieties be included in the thermotropicliquid crystalline polymer, e.g. 6-oxy-2-naphthoyl moiety,2,6-dioxynaphthalene moiety, or 2,6-dicarboxynaphthalene moiety, in aconcentration of not less than about 10 mole percent. The particularlypreferred naphthalene moiety for inclusion in the thermotropic liquidcrystalline polymer is the 6-oxy-2-naphthoyl moiety in a concentrationof not less than about 10 mole percent.

Representative wholly aromatic polyester which exhibit thermotropicliquid crystalline properties include those disclosed in the followingU.S. patents which are herein incorporated by reference: U.S. Pat. Nos.3,991,013; 3,991,014; 4,066,620; 4,067,852; 4,075,262; 4,083,829;4,093,595; 4,118,372; 4,130,545; 4,146,702; 4,153,779; 4,156,070;4,159,365; 4,161,470; 4,169,933; 4,181,792; 4,183,895; 4,184,996;4,188,476; 4,201,856; 4,219,461; 4,224,433; 4,226,970; 4,230,817;4,232,143; 4,232,144; 4,238,598; 4,238,599; 4,238,600; 4,242,496;4,245,082; 4,245,084; 4,247,514; 4,256,624; 4,265,802; 4,267,304;4,269,965; 4,279,803; 4,294,955; 4,299,756; 4,318,841: 4,335,232;4,337,190; 4,337,191; 4,347,349; 4,355,134; 4,359,569; 4,360,658;4,370,466; 4,374,228; 4,374,261; 4,375,530; and 4,377,681.

Representative aromatic-aliphatic polyesters which exhibit thermotropicliquid crystalline properties are copolymers of polyethyleneterephthalate and hydroxybenzoic acid as disclosed in Polyester X-7G-ASelf Reinforced Thermoplastic, by W. J. Jackson, Jr. H. F. Kuhfuss, andT. F. Gray, Jr., 30th Anniversary Technical Conference, 1975 ReinforcedPlastic Composites Institute, The Society of the Plastics Industry,Inc., Section 17-D, Pages 1-4. A further disclosure of such copolymerscan be found in "Liquid Crystal Polymers: I. Preparation and Propertiesof p-Hydroxybenzoic Acid Copolymers", Journal of Polymer Science,Polymer Chemistry Edition, Vol. 14, pages 2043 to 2058 (1976), by W. J.Jackson, Jr. and H. F. Kuhfuss. See also commonly assigned U.S. Pat.Nos. 4,318,842, and 4,355,133. These disclosures are herein incorporatedby reference.

Representative wholly aromatic and aromatic-aliphatic poly(ester-amides)which exhibit thermotropic liquid crystalline properties are disclosedin U.S. Pat. Nos. 4,272,625; 4,330,457; 4,339,375; 4,341,688; 4,351,917;4,351,918 and 4,355,132, which are herein incorporated by reference.

Representative aromatic polyazomethines which exhibit a thermotropicliquid crystalline properties are disclosed in U.S. Pat. Nos. 4,048,148;and 4,122,070. Each of these patents is herein incorporated byreference. Specific examples of such polymers includepoly(nitrilo-2-methyl-1,4-phenylenenitriloethylidyne-1,4-phenyleneethylidyne);poly(nitrilo-2-methyl-1,4-phenylenenitrilo-methylidyne-1,4-phenylene-methylidyne);andpoly(nitrilo-2-chloro-1,4-phenylenenitrilomethylidyne-1,4-phenylene-methylidyne).

Representative aromatic polyester-carbonates which exhibit thermotropicliquid crystalline properties are disclosed in U.S. Pat. Nos. 4,107,143;4,284,757; and 4,371,660 which are herein incorporated by reference.

The anisotropic melt-forming polymer optionally may be blended with oneor more other melt processable polymers which may or may not be capableof forming an anisotropic melt phase provided the resulting blend iscapable of forming the required anisotropic melt phase of the propermelt viscosity. Representative polymer blends which exhibit thermotropicliquid crystalline properties are disclosed in commonly assigned U.S.Pat. Nos. 4,267,289 and 4,276,397, and copending U.S. Ser. Nos. 158,547,filed June 11, 1980 (now U.S. Pat. No. 4,489,190); 165,536, filed July3, 1980 (now U.S. Pat. No. 4,460,735); and 165,532, filed July 3, 1980(now U.S. Pat. No. 4,460,736) which are herein incorporated byreference.

In a preferred embodiment the melt processable anisotropic melt-formingpolymer exhibits a weight average molecular weight of approximately4,000 to 10,000. The weight average molecular weight may be determinedby use of standard gel permeation chromatography. For instance, in atypical test approximately 150 microliters of a 0.1 percent by weightpolymer solution in a solvent consisting of a 1:1 mixture on a volumebasis of pentafluorophenol and hexafluoroisopropanol are introduced intothe gel permeation chromatography equipment consisting of a main controlunit (e.g. a Waters liquid chromatograph Model No. 201), four columnscontaining porous silica particles (e.g. DuPont SE4000, DuPont SE1000,DuPont SE100, and Waters 60 Angstrom Microporasil), and a laser lightscattering unit (e.g. Chromatix KMX6) at ambient temperature. Typicalmelt processable anisotropic melt-forming polymers commonly show aretention time distribution in the range of 20 to 50 minutes.

In order for the melt processable polymer which is selected for use tobe substantially incapable of further chain growth upon heating at itsmelt processing temperature, it is essential that the polymer chainsterminate in functional group which are substantially incapable of afurther polymerization reaction between the adjoining polymer chains.When such polymer is heated in an inert atmosphere (e.g. nitrogen orargon) for 30 minutes while at a temperature of 340° C., its weightaverage molecular weight preferably increases no more than 15 percentand most preferably no more than 10 percent. Accordingly, the polymerdoes not generate to any substantial degree void-forming gaseousby-products when heated and its melt viscosity does not increase to anysubstantial degree upon the passage of time while being heated. Thosethermotropic liquid crystalline polymers customarily formed in the priorart lack this important characteristic. The thermotropic liquidcrystalline polymers of the prior art are routinely formed by techniqueswhereby the requisite reactive groups, which form for example estergroups along the polymer chain, are carefully reacted so as to provide astoichiometric balance of reactive groups. For instance, if a relativelyvolatile monomer, such as hydroquinone or hydroquinone diacetate, isemployed as a reactant, an excess of this monomer sometimes is providedto compensate for the quantity of this reactant which is evolved andlost by volatilization through the use of the specific polymerizationconditions selected. When the various ester-forming monomers areprovided and react with each other under stoichiometrically balancedconditions, a polymer is produced having the random presence of therequisite ester-forming groups at the ends of the polymer chains. Theseend groups unless otherwise end capped in a further reaction step havethe propensity upon subsequent thermal processing (e.g. injectionmolding extruding, compounding, etc.) to react with each other and tocause the polymer chains to continue to grow in length. The thermalprocessing of such polymers to increase the molecular weight in thesolid state is disclosed, for example, in U.S. Pat. Nos. 3,975,487;4,183,895; and 4,247,514. The continued polymerization via acondensation reaction results in the simultaneous evolution oroff-gassing of relatively small molecular by-products and may result ina significant elevation in the melt viscosity of the resulting polymerupon any subsequent melt processing. In accordance with the concept ofthe present invention it is essential that such off-gassing not occur toany substantial degree during encapsulation. The formation of animpervious substantially void-free product is thus assured. Also, arelatively constant melt viscosity is assured which contributes to theuniformity and quality of the resulting encapsulated electronic devices.

According to a possible synthesis technique once the melt processablepolymer which forms an anisotropic melt phase assumes the requisiteweight average molecular weight during its formation via polymerization,the polymer chains may be appropriately end-capped through theintroduction of an end-capping agent to substantially prevent furtherchain growth in the future. For instance, mono-functional end-cappingreactants may be employed.

In accordance with a particularly preferred embodiment of the presentprocess the melt processable polymer which is capable of forming ananisotropic melt phase is formed in accordance with the concept ofcommonly assigned U.S. Ser. No. 517,865, filed July 27, 1983, (now U.S.Pat. No. 4,539,386) of Hyun-Nam Yoon which is entitled "Improved Processfor Forming Thermally Stable Thermotropic Liquid Crystalline Polyesterof Predetermined Chain Length" and which is herein incorporated byreference.

More specifically, when the melt processable polymer is a polyesterwhich optionally may include amide linkages it preferably was formedthrough a polymerization reaction in a polymerization zone ofester-forming and optionally also amide-forming monomers to yield apolymer having recurring moieties selected from the group consisting ofthe following where in each instance Ar comprises at least one aromaticring: ##STR1## where Y is O, NH, or NR, and Z is NH or NR where R is analkyl group of 1 to 6 carbon atoms or an aryl group, ##STR2## where Z isNH or NR where R is an alkyl group of 1 to 6 carbon atoms or an arylgroup, and

(f) mixtures of the foregoing;

and wherein there was provided in the polymerization zone during saidpolymerization reaction an approximately 1 to 4 percent molar excess ofaromatic dicarboxylic acid monomer and/or an esterified derivativethereof which during the polymerization reaction imparted dicarboxyarylunits to the interior of the polymer chains of the resulting polymer andcaused the polymer chains to terminate in carboxylic acid end groupsand/or an esterified derivative thereof wherein the polymer chainsachieved the required molecular weight through the depletion of othermonomers present in the polymerization zone to yield a polyester productwhich was substantially incapable of additional chain growth uponsubsequent heating.

Any of the polyester-forming monomers which are capable of forming apolyester which exhibits an optically anisotropic melt phase may beemployed in such process. Amide-forming monomers optionally mayadditionally be present whereby a poly(ester-amide) is formed whichexhibits an optically anisotropic melt phase. Minor quantities ofcarbonate-forming monomers may be included provided they do notadversely influence the ability of the resulting polyester to exhibit anoptically anisotropic melt phase. In a preferred embodiment theresulting polymer is wholly aromatic in the sense that each moietypresent therein contributes at least one aromatic ring.

As indicated, a possible monomer for selection when forming thepolyester in accordance with such process is one which imparts ##STR3##recurring moieties to the polymer chain where Ar comprises at least onearomatic ring. In a preferred embodiment Ar is 1,4-phenylene or2,6-naphthalene. Accordingly, the moiety is a 4-oxybenzoyl moiety or a6-oxy-2-naphthoyl moiety in such instances. The polyester may include aplurality of different recurring moieties where Ar is different in eachand where each moiety satisfies the above general formula, such as acombination of 1,4-phenylene and 2,6-naphthalene. Such monomers areinherently stoichiometrically balanced since they contain precisely thecorrect quantity of ester-forming reactant groups. The aromatic ring orrings present optionally may include substitution of at least some ofthe hydrogen atoms present thereon. Such substitution may be selectedfrom an alkyl group of 1 to 4 carbon atoms, an alkoxy group of 1 to 4carbon atoms, halogen (e.g. Cl, Br, I), phenyl, and mixtures of theforegoing. Particularly preferred moieties may be derived from4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. Representativering substituted moieties include 2-chloro-4-hydroxybenzoic acid,2,3-dichloro-4-hydroxybenzoic acid, 3,5-dichloro-4-hydroxybenzoic acid,2,5-dichloro-4-hydroxybenzoic acid, 3-bromo-4-hydroxybenzoic acid,3-methyl-4-hydroxybenzoic acid, 3,5-dimethyl-4-hydroxybenzoic acid,2,6-dimethyl-4-hydroxybenzoic acid, 3-methoxy-4-hydroxybenzoic acid,3,5-dimethyl-4-hydroxybenzoic acid, 3-phenyl-4-hydroxybenzoic acid,2-phenyl-4-hydroxybenzoic acid, 6-hydroxy-5-chloro-2-naphthoic acid,6-hydroxy-5-methyl-2-naphthoic acid, 6-hydroxy-5-methoxy-2-naphthoicacid, 6-hydroxy-4,7-dichloro-2-chloro-2-naphthoic acid, etc. Othernon-ring substituted moieties may be derived from 3-hydroxybenzoid acidand 4-hydroxybiphenyl-4'-carboxylic acid.

As indicated, a possible monomer for selection when forming suchpolyester is one which imparts

    --O--Ar--O--

recurring moieties to the polymer chain where Ar comprises at least onearomatic ring. Representative moieties includes: ##STR4## In a preferredembodiment Ar is 1,4-phenylene, 2,6-naphthalene, or 4,4'-biphenyl. Thepolyester may include a plurality of different recurring moieties whereAr is different in each and where each moiety satisfies the abovegeneral formula. The aromatic ring or rings present optionally mayinclude substitution of at least some of the hydrogen atoms presentthereon as discussed in connection with the first described moiety.Examples of moieties which include ring substitution are those derivedfrom phenylhydroquinone, methylhydroquinone, and chlorohydroquinone.Particularly preferred moieties may be simply derived from hydroquinone,2,6-dihydroxynaphthalene, and 4,4'-biphenol.

As indicated, a possible monomer for selection when forming suchpolyester is one which imparts ##STR5## recurring moieties to thepolymer chain where Ar comprises at least one aromatic ring.Representative moieties include: ##STR6## In a preferred embodiment Aris 1,4-phenylene or 2,6-naphthalene the polyester may include aplurality of different recurring moieties where Ar is different in eachand where each moiety satisfies the above general formula. The aromaticring or rings present optionally may include substitution of at leastsome of the hydrogen atoms present thereon as discussed in connectionwith the first described moiety. An example of a moiety which includesring substitution is that derived from phenyl-substituted terephthalicacid. Particularly preferred moieties may be simply derived fromterephthalic acid and 2,6-naphthalenedicarboxylic acid.

As indicated, another possible monomer for selection when forming apolyester in accordance with such process is one which imparts

    --Y--Ar--Z--

recurring moieties to the polymer chain where Ar comprises at least onearomatic ring and where Y is O, NH, or NR, and Z is NH or NR where R isan alkyl group of 1 to 6 carbon atoms or an aryl group. R is preferablya straight chain alkyl group of 1 to 6 carbon atoms and is morepreferably a methyl group. This monomer will impart amide linkages tothe polymer chain. In a preferred embodiment Ar is 1,4-phenylene. Thepolyester may include a plurality of different recurring moieties whereAr is different in each and where each satisfies the above generalformula. The aromatic ring or rings present optionally may includesubstitution of at least some of the hydrogen atoms present thereon asdiscussed in connection with the first described moiety. Examples ofmonomers from which this moiety may be derived include p-aminophenol,p-N-methylaminophenol, p-phenylenediamine, N-methyl-p-phenylenediamine,N,N'-dimethyl-p-phenylenediamine, m-aminophenol, 3-methyl-4-aminophenol,2-chloro-4-aminophenol, 4-amino-1-naphthol, 4-amino-4'-hydroxydiphenyl,4-amino-4'-hydroxydiphenyl ether, 4-amino-4'-hydroxydiphenyl methane,4-amino-4'-hydroxydiphenyl ethane, 4-amino-4'-hydroxydiphenyl sulfone,4-amino-4'-hydroxydiphenyl sulfide, 4,4'-diaminophenyl sulfide(thiodianiline), 4,4'-diaminodiphenyl sulfone, 2,5-diaminotoluene,4,4'-ethylenedianiline, 4,4'-diaminodiphenoxyethane, etc. Particularlypreferred moieties may be derived from p-aminophenol.

As indicated, a further possible monomer for selection when forming apolyester in accordance with such process is one which imparts ##STR7##recurring moieties to the polymer chain where Ar comprises at least onearomatic ring and where Z is NH or NR where R is an alkyl group of 1 to6 carbon atoms or an aryl group. R is preferably a straight chain alkylgroup of 1 to 6 carbon atoms and is more preferably a methyl group. Suchmonomer will impart amide linkages to the polymer chain. These monomersare inherently stoichiometrically balanced since they contain preciselythe correct quantity of ester-forming or amide-forming reactant groups.In a preferred embodiment Ar is 1,4-phenylene. The polyester may includea plurality of different recurring moieties where Ar is different ineach and where each satisfies the above general formula. The aromaticring or rings present optionally may include substitution of at leastsome of the hydrogen atoms present thereon as discussed in connectionwith the first described moiety. Examples of monomers from which thismoiety may be derived include p-aminobenzoic acid,p-N-methylaminobenzoic acid, m-aminobenzoic acid,3-methyl-4-aminobenzoic acid, 2-chloro-4-aminobenzoic acid,4-amino-1-naphthoic acid, 4-N-methylamino-1-naphthoic acid,4-amino-4'-carboxydiphenyl, 4-amino-4'-carboxydiphenyl ether,4-amino-4'-carboxydiphenyl sulfone, 4-amino-4'-carboxydiphenyl sulfide,p-aminocinnamic acid, etc. Particularly preferred moieties may bederived from p-aminobenzoic acid.

Any of the thermotropic liquid crystalline polyesters of the prior artmay be formed in a thermally stable modified form in accordance withsuch process, such as those identified previously. Highly satisfactorypolyesters which may be produced in a modified form in accordance withthe concept of such process are disclosed in commonly assigned U.S. Pat.Nos. 4,161,470; 4,184,996; 4,219,461, 4,256,.624; 4,330,457; and4,351,917; and in commonly assigned copending U.S. Ser. No. 485,820,filed Apr. 18, 1983 (now U.S. Pat. No. 4,473,682). The thermotropicliquid crystalline polyesters of U.S. Pat. Nos. 4,330,457 and 4,351,917additionally include amide linkages.

In accordance with such concept all ester-forming and amide-formingmonomers are added to the polymerization zone in carefully measuredquantities so that during the course of the polymerization there isprovided an approximately 1 to 4 percent molar excess of aromaticdicarboxylic acid monomer and/or an esterified derivative thereof. In apreferred embodiment the aromatic dicarboxylic acid is provided duringthe course of the polymerization reaction in a molar excess ofapproximately 2.0 to 4.2 percent. It is essential that this molar excessof aromatic dicarboxylic acid monomer (and/or an esterified derivativethereof) be provided during the polymerization reaction in excess of theother monomer quantities which are sufficient to provide astoichiometric balance between all carboxylic acid reactive groups(and/or an esterified derivative thereof) and the hydroxyl reactivegroups (and/or an esterified derivative thereof) plus any amine reactivegroups (and/or an amidated derivative thereof).

Preferred aromatic dicarboxylic acid monomers which are provided in thespecified molar excess are terephthalic acid, isophthalic acid,2,6-naphthalene dicarboxylic acid, 2,7-naphthalene dicarboxylic acid,1,4-naphthalene dicarboxylic acid, 2-phenylterephthalic acid,4,4'-bibenzoic acid, etc.

During the course of the polymerization reaction in accordance with suchprocess dicarboxyaryl units derived from such molar excess of aromaticdicarboxylic acid monomer and/or an esterified derivative thereof areincorporated into the interior of the polymer chains of the resultingpolymer and cause the polymer chains to terminate in carboxylic acidgroups and/or an esterfied derivative thereof. As the polymerizationreaction progresses, the other monomers present in the polymerizationzone are fully depleted. The average polymer chain length achieved isdirectly controlled by the molar excess quantity of aromaticdicarboxylic acid monomer and/or its esterified derivative provided inthe polymerization zone during the course of the polymerizationreaction. As the molar excess quantity of dicarboxylic acid monomerand/or its esterified derivative increases within the range specified,the average polymer chain length decreases. As the molar excess quantityof dicarboxylic acid monomer and/or its esterified derivative decreaseswithin the range specified, the average polymer chain length increases.A polymer product of the predetermined average chain length is producedby such process through the selection of the specific molar excessutilized. Such average chain length in many instances may beconveniently evidenced by the inherent viscosity of the resultingpolymer as well as by its weight average molecular weight. In allinstances the polymer chains of the resulting thermotropic liquidcrystalline polyester terminate in carboxylic acid end groups and/or anesterified derivative thereof. Such product is thermally stable sincethe like end groups are substantially incapable of additional polymerchain growth through a polymerization reaction of adjoining moleculesupon subsequent heating at the melt processing temperature.

Such polyester may be formed by a variety of ester-forming techniqueswhereby organic monomer compounds possessing functional groups whichupon condensation form the requisite recurring moieties are reacted. Forinstance, the functional groups of the organic monomer compounds may becarboxylic acid groups, hydroxyl groups, ester groups (e.g. acyloxygroups), acid halides, etc. The organic monomer compounds may be reactedin the absence of a heat exchange fluid via a melt acidolysis procedure.They, accordingly, may be heated initially to form a largely meltsolution of the reactants wherein some reactants such as terephthalicacid initially are present to some degree as solids. Low levels ofterephthalic acid may dissolve under such circumstances. The polymerproduct sometimes is suspended therein as solid polymer particles. Avacuum may be applied to facilitate removal of volatiles formed duringthe final stage of the condensation (e.g., acetic acid or water) and tootherwise expedite the polymerization.

In commonly assigned U.S. Pat. No. 4,067,852 of Gordon W. Calundann,entitled "Melt Processable Thermotropic Wholly Aromatic PolyesterContaining Polyoxybenzoyl Units" is described a slurry polymerizationprocess which may be employed to form such polyester wherein the solidproduct is suspended in a heat exchange medium. The disclosure of thispatent is herein incorporated by reference.

When employing either the melt acidolysis procedure or the slurryprocedure of U.S. Pat. Nos. 4,067,852 the monomer reactants from whichthe polymer moieties are derived which would otherwise include ahydroxyl and/or an amine group are preferably preliminarily esterified.For instance, they may be provided as lower acyl esters of about 2 toabout 4 carbon atoms. Most preferably the acetate esters of suchmonomers which would otherwise include a hydroxyl group and/or an aminegroup are provided. Examples of such reactants are 6-acetoxy-2-naphthoicacid, 4-acetoxybenzoic acid, hydroquinone diacetate, 4,4'-biphenoldiacetate, etc.

Alternatively, any monomers which impart carboxyaryl units to theresulting polymer chain such as the aromatic dicarboxylic acid monomerwhich is provided in molar excess may be provided initially in anesterified form. For instance, they may first be reacted with anaromatic monohydroxy compound such as phenol, m-cresol, p-cresol, etc.as described, for example, in U.S. Pat. No. 4,333,907. Examples of suchreactants are phenyl p-hydroxybenzoate, and diphenyl terephthalate. In apreferred embodiment the carboxylic acid groups of the reactants arenon-esterified.

Representative catalysts which optionally may be employed in either themelt acidolysis procedure or in the procedure of U.S. Pat. No. 4,067,852include dialkyl tin oxide (e.g., dibutyl tin oxide), diaryl tin oxide,titanium dioxide, alkoxy titanium silicates, titanium alkoxides, alkaliand alkaline earth metal salts of carboxylic acids, the gaseous acidcatalysts such as Lewis acids (e.g., BF₃), hydrogen halides (e.g., HCl),etc. The quantity of catalyst utilized typically is about 0.001 to 1percent by weight based upon the total monomer weight, and most commonlyabout 0.01 to 0.2 percent by weight.

The polymerization procedures of commonly assigned U.S. Pat. Nos.4,393,191; 4,395,536; 4,421,908; and 4,429,105 also are suitable for usewhen carrying out such process.

When the melt processable polymer is a modified wholly aromaticpolyester of U.S. Pat. No. 4,161,470, in a particularly preferredembodiment it was formed through a polymerization reaction in apolymerization zone of ester-forming monomers to form a polymer whichconsisted essentially of moieties I and II wherein:

I is ##STR8## and II is ##STR9## wherein said polyester comprisedapproximately 20 to 45 mole percent of moiety I and approximately 55 to80 mole percent of moiety II, and wherein there was provided in thepolymerization zone during said polymerization reaction an approximately1 to 4 percent molar excess of aromatic dicarboxylic acid monomer whichduring the polymerization reaction imparted dicarboxyaryl units to theinterior of the polymer chains of the resulting polymer and caused thepolymer chains to terminate in carboxylic acid end groups wherein thepolymer chains achieved the required molecular weight through thedepletion of other monomers present in the polymerization zone to yielda wholly aromatic polyester product which was substantially incapable ofadditional chain growth upon subsequent heating.

When the melt processable polymer is a modified wholly aromaticpolyester which also includes amide linkages of U.S. Pat. No. 4,330,457,in a further particularly preferred embodiment it was formed through apolymerization reaction of ester-forming and amide-forming reactants toform a polymer which consists essentially of moieties I, II, III, andoptionally IV, wherein in each instance Ar is at least one aromaticring, and wherein:

I is ##STR10## II is ##STR11## III is

    --Y--Ar--Z--,

where Y is O, NH, or NR, and Z is NH or NR where R is an alkyl group of1 to 6 carbon atoms or an aryl group, and

IV is

    --O--Ar--O--,

wherein said poly(ester-amide) comprised approximately 40 to 80 molepercent of moiety I, approximately 5 to 30 mole percent of moiety II,approximately 5 to 30 mole percent of moiety II, and approximately 0 to25 mole percent of moiety IV; and wherein there was provided in thepolymerization zone during said polymerization reaction an approximately1 to 4 percent molar excess of aromatic dicarboxylic acid monomer whichduring the polymerization reaction imparted dicarboxyaryl units to theinterior of the polymer chains of the resulting polymer and caused thepolymer chains to terminate in carboxylic acid end groups wherein thepolymer chains achieved the required molecular weight through thedepletion of other reactants present in the polymerization zone to yielda wholly aromatic poly(esteramide) product which is substantiallyincapable of additional chain growth upon subsequent heating.

When the melt processable polymer which is capable of forming ananisotropic melt phase is formed in accordance with the previouslyreferenced procoss of copending Ser. No. 517,865 (now U.S. Pat. No.4,639,386) of Hyun-Nam Yoon, it preferably additionally exhibits aninherent viscosity of no more than 3.0 dl./g. (and most preferablyapproximately 0.8 to 3.0 dl./g. when dissolved in a concentration of 0.1percent by weight in pentafluorophenol at 60° C. prior to being admixedwith the particulate inorganic material. It should be understood,however, that not all polymeric products will be sufficiently soluble inpentafluorophenol to carry out this inherent viscosity determination.

Other representative techniques for forming the melt processable polymerwhich is capable of forming an anisotropic melt phase, which issubstantially incapable of further chain growth upon heating, and whichmay be employed in the present process are disclosed commonly assigned(1) U.S. Ser. No. 595,004, filed Mar. 29, 1984, (now U.S. Pat. No.4,562,244) of Hyun-Nam Yoon entitled "Improved Process for FormingThermally Stable Thermotropic Liquid Crystalline Polyesters ofPredetermined Chain Length Utilizing Aliphatic Dicarboxylic Acid", and(2) U.S. Ser. No. 611,299 filed May 17, 1984, (now U.S. Pat. No.4,567,247) of Hyun-Nam Yoon entitled "Improved Process for FormingThermally Stable Thermotropic Liquid Crystalline Polyesters ofPredetermined Chain Length Utilizing Aromatic Hydroxyl Monomer and/orAromatic Amine Monomer". The disclosures of these copending applicationsare herein incorporated by reference.

The melt processable polymer will preferably exhibit a melt viscosity ofapproximately 30 to 300 poise at the melt processing temperature (e.g.300° C., 310° C., 320° C., or 330° C.) and at a shear rate of 100 sec.⁻¹prior to admixture with the particulate inorganic material. Such meltviscosity may be determined by standard techniques using an Instroncapillary rheometer having a capillary which measures 4 inches in lengthand has an inner diameter of 30 mils. Alternatively, one may employ aRheometrics mechanical spectrometer to determine melt viscosity usingparallel plates in the steady shear mode at a shear rate of 10 sec.⁻¹.The melt processable polymer should be completely melted at the time ofthe melt viscosity determination. When conducting the melt viscositydetermination at 300° C., a more homogeneous melt can be obtained if thepolymer is first heated to approximately 320° C. and is then cooled to300° C. at which temperature the melt viscosity determination is made.

Regardless of the synthesis route selected the melt processable polymerwhich is selected preferably should be capable of forming the requiredanisotropic melt phase at a temperature in the range of approximately200° to 480° C., and most preferably in the range of approximately 200°to 350° C. prior to being admixed with the particulate inorganicmaterial. The melt processable polymer should be free of deleteriouslevels of contamination and preferably is inherently non-burning inaddition to exhibiting a resistance to light (especially ultravioletlight), solvents, chemicals, and environments which are encounteredduring the formation, assembly, and prolonged use of the encapsulatedelectronic component.

The second essential component of the molding composition in accordancewith the concept of the present invention is the particulate inorganicmaterial (i.e. a mineral filler) which is substantially uniformlydispersed in the melt processable polymer previously described and whichis capable of decreasing the volumetric coefficient of thermal expansionand increasing the thermal conductivity of the melt processable polymer(i.e. of the composition in the absence of the particulate inorganicmaterial). The presence of the particulate inorganic material alsorenders the volumetric coefficient of thermal expansion of the finalproduct to be more isotropic in nature. While the particulate materialmay be selected from a wide variety of solid inorganic substances, suchas silicon dioxide, talc, wollastonite, alumina, cordierite, etc.,certain forms of silicon dioxide are preferred. The weight averageparticle size of the particulate inorganic material is preferablyapproximately 1 to 50 microns with at least 99 percent by weight of theparticles being below 100 microns. Suitable particle size analyzers forus when making such particle size determination are available fromMicrometrics Instrument Corporation of Norcross, Georgia and the Leedsand Northrup Corporation of St. Petersburg Florida (Microtrac particlesize analyzer). Such material also preferably exhibits substantially thesame dimensions in all directions so as to eliminate the possiblecreation of anisotropy in thermal expansion following the molding of theparticles in an aligned configuration. Accordingly, the particulateinorganic material preferably has an average aspect ratio of no morethan 2:1 as determined by conventional optical microscopy.

It is essential that the particulate inorganic material be free ofdeleterious levels of contamination which would harm or interfere withthe operation of the electronic component, and to preferably have as lowa volumetric coefficient of thermal expansion as possible. Accordingly,fused silica is the particularly preferred particulate inorganicmaterial for use in the present invention. As is well known, fusedsilica is composed of a relatively pure form of silicon dioxide whichhas been converted through the use of very high temperatures commonlyinvolving electric arc melting from its usual crystalline to anamorphous form. Such inorganic particulate material is sometimes calledfused quartz. The resulting fused particles following their formationare ground to the desired particle size. Such material exhibits avolumetric coefficient of thermal expansion which is practically zero,and can undergo rapid and extreme temperature changes without creatinginternal stresses. Such fused silica is commercially available, and maybe obtained from Harbison-Walker Refractories of Pittsburgh,Pennsylvania, under the designation GP7I. Minor amounts of crystallinesilica may be blended with the fused silica in order to further increaseits thermal conductivity if desired. The presence of crystalline silicawill, however, tend to increase to some degree the volumetriccoefficient of thermal expansion of the overall composition. In someapplications where the electronic component undergoing encapsulation isnot particularly fragile, discontinuous glass fibers or other similarfibrous reinforcement may be included with the particulate inorganicmaterial. Also, colorants, additives, adhesion promoters, lubricants,etc. may be included so long as they do not deleteriously influence thecomposition.

In a preferred embodiment the particulate inorganic materialadditionally bears a coating upon its surface which increases itsability to admix with the melt processable polymer which is capable offorming an anisotropic melt phase. Any coating selected must beincapable of harming the electronic component or interfering with itsoperation during use. Representative coatings are the silanes such asgamma-glycidoxypropyltrimethoxysilane andgamma-aminopropyltriethoxysilane available from the Union CarbideCorporation under the designations A187 and A1100 respectively. Suchsilanes may be applied as surface coatings to the particulate inorganicmaterial in concentrations of approximately 0.5 to 1.5 percent by weightin accordance with standard coating technology for mineral fillers.Organotitanate surface coatings may be similarly applied to theinorganic particles prior to blending with the melt processablepolymeric material which is capable of forming an anisotropic meltphase.

In accordance with the concept of the present invention the particulateinorganic material is blended with and substantially uniformly dispersedwithin the melt processable polymer which is capable of forming ananisotropic melt phase in a concentration of approximately 40 to 80percent by weight based upon the total weight of the moldingcomposition, and most preferably in a concentration of approximately 50to 75 percent by weight (e.g. approximately 55 to 70 percent by weight)based upon the total weight of the molding composition. Suchsubstantially uniform dispersal may be accomplished by known techniqueswherein the particulate material is forced within the moving moltenpolymeric material. Known melt compounding techniques using single screwextruders, co-rotating twin screw extruders, counter-rotating twin screwextruders, kneaders, etc. may be employed. For instance, a co-rotatingtwin screw extruder manufactured by Werner & Pfleiderer Corporation ofRamsey, New Jersey may be employed. When using such equipment, preformedpolymer pellets and the particulate inorganic material may be simply fedas a dry blend into the extruder and heated to above the meltingtemperature of the polymeric material. Kneading blocks advantageouslymay be included within the screw to aid in the blending. Whenintroducing the particulate inorganic material at high concentrationlevels, a multiple pass (e.g. a two or more pass) blending operation maybe employed with only a portion of the particulate inorganic materialbeing introduced during the first pass. Alternatively, the blend ofanisotropic melt-forming polymer and the particulate inorganic materialcan be prepared by adding the entire volume of inorganic material to themolten polymer by feeding the particles into one or more feed portslocated downstream on a compounding extruder, such as a Buss-ConduxKneader manufactured by Buss-Condux of Elk Grove Village, Ill., or otherextruder capable of downstream addition of particles. Using thisprocedure, the polymer is fed into the rear of the extruder, melted, andthen blended with the particles. With either type of blending process,the resulting molding composition may be pelletized using either astrand or a dieface pelletizing procedure. With a strand procedure, astrand of the resulting molding composition may be extruded andpelletized following passage through an air quench. The melt extrudedstrand may be transported on a conveyor between the extruder and thepelletizing operation. With a die-face cutter, the molding compositionmay be cut into pellets at the face of the die, with the pellets thenbeing dropped into water to cool.

The resulting molding composition commonly is capable of being injectionmolded at a temperature within the range of approximatley 250° to 390°C. Also, the resulting molding composition preferably includes less than50 parts per million of water-extractable alkali metal (e.g. Na and K),and less than 100 parts per million of water-extractable halogen (e.g.Cl, Br, F). Accordingly, charged species are not present which wouldinterfere with the delicate electrical balance ofte required during theoperation of an encapsulated electronic component of some types. Thequantity of water-extractable alkali metals and halogens present may bedetermined by following the Recommended Practice for Aqueous Extractionof Ionic Species (Section G5.3) in the Book of SEMI Standards publishedby the Semiconductor Equipment and Materials Institute, Vol. 4,Packaging Division (1983).

When carrying out the encapsulation of an electronic component inaccordance with the concept of the present invention, the preformedelectronic component may be positioned (i.e. secured) within a moldcavity in a manner directly analogous to that employed in priorinjection packaging techniques (including transfer molding methods) forsuch components. If desired, a plurality of electronic components may bepositioned in a multicavity mold where each electronic component isindividually encapsulated. The molding composition while at an elevatedtemperature next is caused to completely fill the mold cavitysurrounding the electronic component while the melt processable polymeris in the molten state and the particulate inorganic material isdispersed therein. It should be understood, however, that predeterminedportions of the electronic component, such as its electrical contacts,etc., may extend outside the area which is surrounded by the moldingcomposition.

As the liquid crystalline polymer of the molding composition is injectedwithin the mold cavity, the molecules thereof are believed inherently totend to locally orient in a manner which ultimately imparts strength andstiffness to the encapsulant in spite of its relatively low molecularweight. The flow of the molten polymer readily causes its orientation.Such local orientation is not lost to any significant degree prior tosolidification because of the very long relaxation times associated withsuch polymers. Accordingly, the mechanical properties of the resultingarticle are advantageously influenced by the thermotropic properties ofthe molten polymer present in the molding composition.

The molding composition preferably exhibits a melt viscosity within therange of approximately 300 to 2500 poise at the injection moldingtemperature while at a shear rate of 100 sec.⁻¹, and most preferablyexhibits a melt viscosity within the range of approximately 300 to 1500poise under such conditions.

An alternate technique for assessing the moldability characteristics ofthe molding composition is described in the Spiral Flow Test of ASTMD3123-72 as modified by the use of a conventional injection moldingmachine, a spiral flow mold having a 1/4 inch diameter one-half roundgeometry with a 50 inch flow length, a mold temperature of 100° C., andan injection pressure of 8000 psi. Typical flow lengths for the moltenpolymer obtained under these conditions commonly range fromapproximately 10 to 45 inches. The polymers which exhibit the longerflow lengths are more suitable for encapsulating delicate electroniccomponents.

The rate at which the remaining space in the mold cavity is filled bythe molding composition will be influenced by the size and structuralproperties of the electronic component. Mold fill times of approximately2 to 15 seconds commonly are selected within a total molding cycle ofless than one minute (e.g. approximately 15 to 50 seconds). Oftenrelatively delicate electronic components, such as integrated circuitdevices, require a substantially slower filling rate than a more ruggedelectronic components, such as a single-functioned transistor. Care istaken not to deleteriously deflect portions of the electronic componentundergoing encapsulation. The minimum mold cavity fill time preferablyis selected in each instance which is found not to damage the electroniccomponent.

In order to aid in the complete filling of the mold cavity with themolding composition and to thereby aid in the creation of an imperviousencapsulation, the mold itself preferably also is provided at anelevated temperature while it is being filled. Accordingly, the moldingcomposition is prevented from cooling unduely prior to the completefilling of the mold cavity. The choice of the temperature of the moldand the temperature of the molding composition during the mold fillingstep will be influenced by the melting temperature of the meltprocessable polymer which is capable of forming an anisotropic meltphase and the temperature required to achieve a melt viscosity whichwill completely fill the mold cavity with ease. The melt viscosity willin turn be influenced by the concentration and the particle sizedistribution of the particulate inorganic material dispersed in themolding composition and the weight average molecular weight of the meltprocessable polymer. Optimum conditions for a given encapsulation runwithin the parameters stated can be determined by routineexperimentation and are influenced by the gate size, runner length andsize, and other geometric factors related to the design of the specificmold being utilized. Commonly, the mold cavity is provided at atemperature of approximately 100° to 250° C. and the molding compositionis introduced into the mold cavity at a temperature of approximately250° to 390° C. while under a pressure of approximately 100 to 1000 psi.

Representative apparatus for carrying out the encapsulation include (1)a 1 or 2 ounce, 35 or 40 ton clamp capacity Arburg Model 220 screwinjection molding machine, and (2) a 5 ounce, 80 ton clamp capacityWindsor Model HSI 80 screw injection molding machine. A processcontroller can be used to advantage to provide positive feedbackposition control of the injection molding ram and to thereby control thefilling rate in view of the relatively low pressures commonly employed.The use of short runners and generously proportioned gates isrecommended in order to reduce the pressure during flow and to ease themolding process.

Once the mold is completely filled with the molding composition, themolding composition solidifies therein to form an impervious packagearound the desired portion of the electronic component. The polymeremployed is thermally stable and does not evolve to any substantialdegree volatile voidforming components during the molding operation,such as would occur if substantial further polymerization or degradationtook place. The melt viscosity of the molding composition remainssubstantially constant. The creation of an impervious encapsulation isassured. Additionally, no polymer curing step is required subsequent tothe solidification of the polymer in the mold cavity.

Following the injection molding of the molding composition it preferablyexhibits a volumetric coefficient of thermal expansion of no more than150×10⁻⁶ cm³ /cm.³ °C. at 60 to 110° C., and most preferably no morethan 90×10⁻⁶ cm.³ /cm³ °C. at such conditions. This is important sincemost electronic devices generate som heat during operation and theresulting thermally induced stresses if too extreme may cause crackingof the previously impervious encapsulant resulting in its failure. Also,bonding wires may be pulled from integrated circuit pads, wire bonds maybe broken due to fatigue, or circuit conductors may loosen and be brokenfrom the surface of the integrated circuit chip. The lesser values areachieved with the inclusion of the greater quantities of the particulateinorganic material in the molding composition. The particulate inorganicmaterial renders the volumetric coefficient of thermal expansion of thepolymeric material to be more isotropic in nature. Such volumetriccoefficient of thermal expansion may be determined by the Standard TestMethod for Expansion Characteristics of Molding Compounds (Section G5.4)in the Book of SEMI Standards published by the Semiconductor Equipmentand Materials Institute, Vol. 4, Packaging Division (1983).

The molding composition of the present invention following injectionmolding exhibits good thermal expansion properties over a relativelybroad temperature range (e.g. from -40° C. to 150° C.). These propertiestend to be substantially better than those exhibited by epoxycompositions which show an increased thermal expansion above the glasstransition temperature.

Following the injection molding of the molding composition it preferablyexhibits a thermal conductivity of at least 10×10⁻⁴ cal.-cm./sec.cm.²°C., and most preferably of at least 13×10⁻⁴ cal.-cm./sec.cm.² °C. Thisis important since high temperatures may deleteriously affect theperformance of some electronic components. For instance, heat is knownto slow the speed at which integrated circuits operate. The heatgenerated must be effectively dissipated. The higher thermalconductivity values ar achieved with the inclusion of larger quantitiesof the particulate inorganic material in the molding composition. Thethermal conductivity may be determined by standard techniques commonlyemployed in the industry.

Following the injection molding of the molding composition it alsopreferably exhibits a V-0 burning rating when subjected to the UL-94test. When subjected to such UL-94 test the molded article shouldpossess a thickness of at least 30 mils. It preferably exhibits goodhydrolytic stability as evidenced by a retention of at least 75 percentof the flexural strength thereof following 200 hours in water at 110° C.The encapsulated electronic component additionally will preferablyexhibit no change in its electrical characteristics following heating at85° C. for 1000 hours in air of 85 percent relative humidity.

The encapsulation made possible by the present invention is consideredto be impervious in the sense that it is substantially void free and iscapable of well protecting the electronic component from liquids andgases which are encountered during service. Moisture cannot penetratethe bulk of the device or travel to the interior of the device bycapillary action along leads which extend beyond the encapsulated area.The electronic component is well protected from ultraviolet light.

The mechanical properties (including the flexural strength) of themolding composition following injection molding are sufficient to resistthe mechanical stresses involved during trimming and forming and whileincorporating the device into a finished assembly. Since products may beformed of highly consistent geometry, they are capable of being insertedin sockets or in printed circuit boards via automation without damage.

The mechanical properties of the molding composition following injectionmolding are also sufficient to withstand the trimming of waste metalfrom the lead frame and/or the bending of leads at right angles whichextend beyond the encapsulated area. Accordingly stresses exerted at theedges of the encapsulated area of the electronic component may beeffectively withstood in the absence of microcracking which wouldotherwise impair the useful life of the electronic component. The longterm performance of the electronic component on a reliable basis isassured by the presence of the encapsulant of the present invention.

The following examples are presented as specific illustrations of theclaimed invention. It should be understood, however, that the inventionis not limited to the specific details set forth in the examples.

EXAMPLE I

The melt processable polymer capable of forming an anisotropic meltphase selected for use was that formed in accordance with Example II ofcommonly assigned U.S. Ser. No. 517,865, filed July 27, 1983 now U.S.Pat. No. 4,539,386), of Hyun-Nam Yoon entitled "Improved Process forForming Thermally Stable Thermotropic Liquid Crystalline Polyesters ofPredetermined Chain Length".

More specifically, to a 50 gallon stainless steel reactor equipped witha sealed anchor stirrer, gas inlet tube, and distillation columnconnected to a condenser were added at room temperature (i.e.approximately 25° C.) the following:

(a) 115 pounds of 6-acetoxy-2-naphthoic acid (0.50 pound mole),

(b) 130.2 pounds of 4-acetoxybenzoic acid (0.745 pound mole),

(c) 4.46 pounds of terephthalic acid (0.0268 pound mole), and

(d) 6.98 grams of potassium acetate catalyst.

It can be calculated that a molar excess of 2.15 percent of terephthalicacid monomer was provided in the reactor. The 6-acetoxy-2-naphthoic acidand 4-acetoxybenzoic acid reactants where inherently stoichiometricallybalanced since each provided the required carboxylic acid and acetoxyester-forming reactant groups in an identical quantity. Accordingly, theterephthalic acid monomer served as an aromatic dicarboxylic acidmonomer and provided the ester-forming carboxylic acid groups in astoichiometric excess beyond the stoichiometric balance which existedwith respect to the other monomers present.

The reactor and its contents were thoroughly purged of oxygen byevacuating and refilling with nitrogen three times, and hot oil was nextcaused to flow through the jacket of the reactor which caused thereactants to melt. The contents of the reactor were heated to 208° C.and were maintained at that temperature for 118 minutes. In 15 minuteincrements the contents of the reactor were next heated to the followingtemperatures: 213° C., 220° C., 234° C., 246° C., 259° C., 273° C., 290°C., and 303° C. Then the temperature was raised to 325° C. in 47minutes.

When the reactant temperature reached 325° C. during theabove-identified heating schedule, a vacuum of 8 mm. Hg was applied tothe reactants while heating continued. Such heating under vacuumcontinued for 90 minutes. The vacuum next was broken with nitrogen andthe molten polymer product was discharged through a 1/8 inch, three-holedie, immersed in water to form solidified strands, and was pelletized.Approximately 150 pounds of the wholly aromatic polyester product wereobtained.

The chains of the resulting polymer included 1,4-dicarboxyphenyleneunits at interior locations along the length of the polymer chains andterminated in carboxylic acid end groups. When the polymer was heated ineither the melt or in the solid phase, no substantial furtherpolymerization or chain growth was observed.

The inherent viscosity (I.V.) of the polymer product was found to be 1.6dl./g. as determined in a pentafluorophenol solution of 0.1 percent byweight concentration at 60° C. in accordance with the equation: ##EQU1##where c=concentration of solution (0.1 percent by weight), andηrel=relative viscosity. The weight average molecular weight wasapproximately 9,700. When the polymer was subjected to differentialscanning calorimetry (20° C./min. heating rate), it exhibited a meltendotherm peak at 236° C. The polymer melt was optically anisotropic andexhibited a melt viscosity of approximately 50 poise at a temperature of300° C. and a shear rate of 100 sec.⁻¹.

The particulate inorganic material selected for use was fused silicawhich was purchased from Harbison-Walker Refractories of Pittsburgh,Pa., under the GP7I designation. The aspect ratio of such material wassubstantially 1:1, and its weight average particle size wasapproximately 12 microns with more than 99 percent by weight of theparticles being below 100 microns. Such fused silica additionally wassurface treated with a 1 percent by weight coating ofgammaglycidoxypropyltrimethoxysilane using standard coating technologyfor mineral fillers. This silane coating was available from the UnionCarbide Corporation under the A187 designation.

The fused silica was substantially uniformly dispersed in a portion ofthe wholly aromatic polyester which was capable of forming ananisotropic melt phase in a concentration of 70 percent by weightthrough the use of a compounding extruder, Model MDK 46 Kneader whichwas manufactured by Buss-Condux of Elk Grove Village, Ill. The polymerpellets were fed into the rear of the extruder. The fused silica wasmetered into the second feed port downstream on the extruder. The barreltemperatures were maintained at 250° C. A screw rotation of 300 RPM wasemployed. The blended material was fed out of a 3-hole die with holediameters of 4.5 mm. at a rate of approximately 22 lbs./hr. and waschopped into pellets using a single blade eccentric die-face pelletizer.The pellets were then sprayed with water to cool the material to roomtemperature.

The resulting molding composition exhibited a melt viscosity of 900poise at 330° C. (i.e. the approximate encapsulation temperature to beused later) while under a shear rate of 100 ; sec.⁻¹. The moldingcomposition also included less than 50 parts per million ofwater-extractable alkali metal, and less than 100 parts per million ofwater-extractable halogen.

Precision wire wound resistors may be selected for encapsulation. Thewound wire portion of the resistors is provided on a coil which iscapable of resisting deformation at 350° C. On each resistor a pair ofaxially disposed leads are attached in a conventional manner topretinned copper lead wires of 24 gauge (i.e. 0.020 inch diameter) bysecure bonds which adhere well at 350° C. To aid in handling, aplurality of the resistors to be encapsulated may be mounted in a spacedside-by-side parallel relationship on a tape provided with adhesive andwound on an appropriate supply reel.

The injection molds selected possess dimensions sufficient to permit thecomplete encapsulation of the wire wound resistors including the leadsand bonds which secure the copper lead wires to the leads. The internalmold dimensions are approximately 0.060 inch larger than the electroniccomponent on all sides. The lead wires extend outwardly throughthermally resistant slightly compliant seals which form portions of themold walls. The mold cavities possess about a 5 degree draft angle toaid in the removal of the electronic components following encapsulation.A plurality of the mold cavities are provided in a side-by-siderelationship in order to enable the simultaneous encapsulation of aplurality of the electronic components.

A Model 200S screw injection molding machine manufactured by Arburg anddistributed by Polymer Machinery Corporation of Berlin, Conn., may beused to introduce the molding composition into the mold cavities. Thein3ection molding machine has a 40 ton clamp and a 2 oz. shot size. Theencapsulant may be introduced into the mold cavities by means of a 1/4inch full round runner through single gates which are 0.125 inch wideand 0.020 inch high with substantially no land. While the moldingcomposition is at a temperature of approximately 330° C. and at apressure of about 500 psi. the mold cavities may be filled inapproximately one second while the mold cavities are maintained atapproximately 125° C. Within a few seconds the molding compositionsolidifies in each mold cavity. The encapsulated electronic componentmay be ejected from each mold cavity by means of a 0.125 inch ejectionpin and is deflashed.

The resulting injection molded molding composition will exhibit a V-0burning rating when subjected to the UL-94 test, a volumetriccoefficient of thermal expansion of no more than 150×10⁻⁶ cm.³ /cm.³ °C.at 60° to 110° C., a thermal conductivity of at least 10×10⁻⁴ cal.cm./sec. cm.² °C., and hydrolytic stability as evidenced by a retentionof at least 75 percent of the flexural strength thereof following 200hours in water at 110° C.

EXAMPLE II

To a 50 gallon stainless steel reactor equipped with a sealed anchorstirrer, gas inlet pipe, and distillation column connected to acondenser were added at room temperature (i.e., approximately 25° C.)the following:

(a) 115 pounds of 6-acetoxy-2-naphthoic acid (0.50 pound mole),

(b) 126 pounds of 4-acetoxybenzoic acid (0.70 pound mole),

(c) 8.29 pounds of terephthalic acid (0.050 pound mole), and

(d) 5.65 grams of potassium acetate catalyst.

It can be calculated that a molar excess of 4.17 percent of terephthalicacid monomer was provided in the reactor. The 6-acetoxy-2-naphthoic acidand 4-acetoxybenzoic acid reactants were inherently stoichiometricallybalanced since each provided the required carboxylic acid and acetoxyester-forming reactant groups in an identical quantity. Accordingly, theterephthalic acid monomer served as an aromatic dicarboxylic acidmonomer and provided the ester-forming carboxylic acid groups in astoichiometric excess beyond the stoichiometric balance which existedwith respect to the other monomers present.

The reactor and its contents were thoroughly purged of oxygen byevacuting and refilling with nitrogen three times, and hot oil was nextcaused to flow through the jacket of the reactor which caused thereactants to melt. The contents of the reactor were heated to 200° C.and were maintained at that temperature for 100 minutes. In 15 minuteincrements the contents of the reactor were next heated to the followingtemperatures: 231° C., 244° C., 262° C., 273° C., 292° C., 306° C., 311°C., and 320° C. Then the temperature was maintained at 320° C. for 35minutes.

After the reactant temperature was maintained at 320° C. during theabove-identified heating schedule, a vacuum of 8 mm. Hg was applied tothe reactants while heating continued. Such heating under vacuumcontinued for 60 minutes. The vacuum next was broken and the moltenpolymer product was discharged through a 1/8 inch, one-hole die,immersed in water to form solidified strands, and was pelletized.Approximately 138 pounds of the wholly aromatic polyester product wereobtained.

The chains of the resulting polymer included 1,4-dicarboxyphenyleneunits at interior locations along the length of the polymer chains andterminated in carboxylic acid end groups. When the polymer was heated ineither the melt or in the solid phase, no substantial furtherpolymerization or chain growth was observed.

The inherent viscosity (I.V.) of the polymer product was found to be0.99 dl./g. as determined in a pentafluorophenol solution of 0.1 percentby weight concentration at 60° C. as previously described. The weightaverage molecular weight was approximately 6,100. When the polymer wassubjected to differential scanning calorimetry (20° C./min. heatingrate), it exhibited an endotherm peak at 221° C. The polymer melt wasoptically anisotropic and exhibited a melt viscosity of approximately 20poise at a temperature of 300° C. and a shear rate of 100 sec.⁻¹.

The polymer was next blended with fused silica as described in Example Ito form a molding composition in accordance with the present invention.The resulting molding composition exhibited a melt viscosity of 420poise at 330° C. while under a shear rate of 100 sec.⁻¹. The moldingcomposition also included less than 50 parts per million ofwater-extractable alkali metal, and less than 100 parts per million ofwaterextractable halogen.

A presoldered 16 pin lead frame with dual-in-line integrated circuitdevices may be selected for encapsulation. The lead frame strip isplated with a eutectic tin-lead alloy in a 63/37 tin/lead ratio andcontains 10 integrated circuits in a row and measures 1 inch×7.5inches×0.006 inch in thickness. The integrated circuit dies measureapproximately 1/4×1/4 inch and are cemented to the paddle portion ofeach of the 10 devices by use of an epoxy adhesive. Each of the 16 pinsof each device are attached to the aluminum pads of the integratedcircuit dies by delicate gold wires having a diameter of approximately0.001 inch.

An Engel Model ES50 VHAS 85 ton vertical opening press manufactured byLudwig Engel Canada, Ltd., of Guelph, Ontario, Canada, may be selectedto accomplish the desired encapsulation with the aid of an integralprocess controller. The injection mold selected has a plurality ofcavities connected to central 1/4 inch round runners which are fed atthe parting line from a horizontal injection cylinder. The dimensions ofthe individual mold cavities measure approximately 0.75inch×approximately 0.25 inch×approximately 0.117 inch.

The lead frame may be positioned in the mold with the aid of guide pinsto position each integrated circuit die at the center of mold cavity.The mold cavities possess about a 5 degree draft angle to aid in theremoval of the electronic components following encapsulation. Themolding composition is prevented from leaving the mold between the leadswhich extend to the outside by "dam bars" or webs positioned betweeneach lead.

Molding conditions may be selected to accommodate the presoldered leadframe. With a melt temperature of approximately 315° C. and at apressure of approximately 300 psi, the mold cavities surrounding theelectronic components may be completely filled over a period ofapproximately 3.8 seconds. During such introduction of the moldingcomposition, the mold cavity may be held at a temperature ofapproximately 145° C. Once present in the mold cavity the moldingcomposition solidifies in a few seconds. The encapsulated electroniccomponents may be ejected from cavities by means of 0.150 inch diameterejection pins.

Following encapsulation the "dam bars" or webs may be removed in atrimming step wherein all of the surplus metal of the lead frame whichextends outside the encapsulated area is removed by die cutting. Theleads which extend outside the encapsulated area may be bent to theirfinal configuration which facilitates insertion in a socket or in aprinted circuit board. The electronic component is imperviouslyencapsulated.

The resulting injection molded molding composition will exhibit a V-Oburning rating when subjected to the UL-94 test, a volumetriccoefficient of thermal expansion of no more than 150×10⁻⁶ cm.³ /cm.³ °C.at 60 to 110° C., a thermal conductivity of at least 10×10⁻⁴ cal.cm./sec. cm ² °C., and hydrolytic stability as evidenced by a retentionof at least 75 percent of the flexural strength thereof following 200hours in water at 110° C.

EXAMPLE III

To a 50 gallon stainless steel reactor equipped with a sealed anchorstirrer, gas inlet pipe, and distillation column connected to acondenser were added at room temperature (i.e. approximately 25° C.) thefollowing:

(a) 59.0 pounds of 6-hydroxy-2-naphthoic acid (0.31 pound mole),

(b) 137.4 pounds of 4-hydroxybenzoic acid (1.00 pound mole),

(c) 9.04 pounds of terephthalic acid (0.054 pound mole), and

(d) 142 pounds of acetic anhydride (1.39 pounds mole), and

(e) 5.6 grams of potassium acetate catalyst.

It can be calculated that a molar excess of 4.1 percent of terephthalicacid monomer was provided in the reactor. The 6-hydroxy-2-naphthoic acidand 4-hydroxybenzoic acid reactants where inherently stoichiometricallybalanced since each provided the required carboxylic acid and hydroxyester-forming reactant groups in an identical quantity. Accordingly, theterephthalic acid monomer served as an aromatic dicarboxylic acidmonomer and provided the ester-forming carboxylic acid groups in astoichiometric excess beyond the stoichiometric balance which existedwith respect to the other monomers present.

The reactor and its powdery contents wer thoroughly purged of oxygen byevacuating and refilling with nitrogen three times, the acetic anhydridewas introduced, and hot oil was next caused to flow through the jacketof the reactor which caused the reactants to form a homogeneous liquidsolution. The contents of the reactor were heated to 140° C., weremaintained at that temperature for 30 minutes, were heated to 200° C. inapproximately 40 minutes, and were maintained at 200° C. for another 30minutes. In 15 minute increments the contents of the reactor were nextheated to the following temperatures: 219° C., 246° C., 262° C., 281°C., 300° C., 310° C., 316° C., and 320° C. Then the temperature wasmaintained at 320° C. for 30 minutes.

After the reactant mixture was maintained at 320° C. during theabove-identified heating schedule, a vacuum of 10 mm. Hg was applied tothe reactants while heating continued. Such heating under vacuumcontinued for 120 minutes. The vacuum next was broken with nitrogen andthe molten polymer product was discharged through a 1/8 inch, one-holedie, immersed in water to form solidified strands, and was pelletized.Approximately 150 pounds of the wholly aromatic polyester product wereobtained.

The chains of the resulting polymer included 1,4-dicarboxyphenyleneunits at interior locations along the length of the polymer chains andterminated in carboxylic acid end groups. When the polymer was heated ineither the melt or in the solid phase, no substantial furtherpolymerization or chain growth was observed.

The inherent viscosity (I.V.) of the polymer product was found to be 0.9dl./g. as determined in a pentafluorophenol solution of 0.1 percent byweight concentration at 60° C. as previously described. The weightaverage molecular weight was approximately 6,000. When the polymer wassubjected to differential scanning calorimetry (20° C./min. heatingrate), it exhibited a melting range from approximately 250° to 305° C.The polymer melt was optically anisotropic and exhibited a meltviscosity of approximately 7 poise at a temperature of 320° C. and ashear rate of 10 sec.⁻¹ where measured between the parallel plates of aRheometrics mechanical spectrometer operating in the steady shear mode.

The polymer was next blended with fused silica as described in Example Ito form a molding composition in accordance with the present invention.The resulting molding composition was evaluated using the spiral flowtest of ASTM D3123-72 as modified as previously described. At 330° C. aspiral flow length of 24 inches was obtained, and at 340° C. a spiralflow length of 28 inches was obtained. The molding composition alsoincluded less than 50 parts per million of water-extractable alkalimetal, and less than 100 parts per million of water extractable halogen.

A 40 pin lead frame dual-in-line integrated circuit device may beselected for encapsulation. The lead frame consists of plurality ofsegments which measure 11/8 inch by 21/4 inches and is composed of a0.010 inch prestamped copper sheet. The integrated circuit die measuresapproximately 1/4×1/4 inch and is cemented to the paddle portion of eachsegment of the lead frame by use of an epoxy resin. Each of the 40 pinsof the lead frame are attached to the pads of the integrated circuit dieby delicate gold wires having a diameter of approximately 0.001 inch.

The injection mold selected possesses dimensions to permit the completeencapsulation of the integrated circuit die, the connecting wires, andthe associated cantilevered arms of the lead frame. Each half of themold cavity measures 2.03 inches in length, and 0.54 inch in width. Thetotal thickness of the two halves of the mold cavity measures 0.165 inchwhich includes the 0.006 inch thickness of the lead frame over whicheach half of the mold cavity is placed. The mold is designed toincorporate a generous draft angle of about 5 degrees to ease theejection of the encapsulated electronic component. The lead frame issecured in the mold by guide pins which mate with holes stamped in thelead frame to center the die in the cavity. The molding composition isprevented from leaving the mold between the leads which extend outsidethe mold "dam bars" or webs positioned between each lead.

An Engel Model ES50 VHAS 85 ton vertical press manufactured by LudwigEngel Canada, Ltd. of Guelph, Ontario Canada, may be used to introducethe molding composition into the multicavity mold with the aid of anintegrated process controller. The injection molding machine has an 85ton clamp and a 5 oz. capacity. The encapsulant is introduced into themold cavities via parting line injection into 1/4 inch runners leadingto a single gate for each mold cavity. The gates which measureapproximately 0.125 inch×0.035 inch and have substantially no land arelocated at the center of each mold cavity on one side. While at a melttemperature of approximately 330° C. and an injection pressure on themolten polymer of approximately 1000 psi, all mold cavities may befilled through the gates over a period of approximately 21/2 seconds.During such introduction of the molding composition, the mold cavitiesmay be provided at a temperature of approximately 175° C. Once presentin the mold cavity the molding composition solidifies within a fewseconds. The electronic component may be ejected from the mold cavity bymeans of an ejection pin having a diameter of approximately 5/32 inch.

Following encapsulation the "dam bars" or webs may be removed in atrimming step wherein all of the surplus metal of the lead frame whichextends outside the encapsulated area is removed by cutting. The leadswhich extend outside the encapsulated area may be bent to their finalconfiguration which facilitates insertion in a socket or in a printedcircuit board. The electronic component is imperviously encapsulated.

The resulting injection molded molding composition will exhibit a V-0burning rating when subjected to the UL-94 test, a volumetriccoefficient of thermal expansion of no more than 150×10⁻⁶ cm.³ /cm.³ °C. at 60 to 110° C., a thermal conductivity of at least 10×10⁻⁴ cal. cm/sec. cm² ° C., and hydrolytic stability as evidenced by a retention ofat least 75 percent of the flexural strength thereof following 200 hoursin water at 110° C.

Although the invention has been described with preferred embodiments itis to be understood that variations and modifications may be employedwithout departing from the concept of the invention defined in thefollowing claims.

We claim:
 1. A method for imperviously encapsulating on a relativelyexpeditious basis an electronic component using a novel compositionincorporating a thermoplastic polymer which forms an anisotropic meltphase comprising:(1) introducing the electronic component to beencapsulated within a mold cavity, (2) completely filling the moldcavity surrounding said electronic component by injection at an elevatedtemperature of a molding composition comprising (a) a moltenthermoplastic melt processable polymer which is capable of forming ananisotropic melt phase, has a weight average molecular weight ofapproximately 4,000 to 25,000, and which is substantially incapable offurther chain growth upon heating as evidenced by an increase in theweight average molecular weight of no more than 15 percent when heatedin an inert atmosphere for 30 minutes while at a temperature of 340° C.,and (b) approximately 40 to 80 percent by weight based upon the totalweight of the molding composition of a particulate inorganic materialsubstantially uniformly dispersed in component (a) which is capable ofdecreasing the volumetric coefficient of thermal expansion andincreasing the thermal conductivity of component (a), (3) cooling thecontents of the mold cavity to allow said molding composition tosolidify and to form an impervious package around said electroniccomponent, and (4) removing said resulting imperviously encapsulatedelectronic component from said mold cavity.
 2. A method for imperviouslyencapsulating an electronic component according to the method of claim 1wherein said electronic component is a semiconductor device.
 3. A methodfor imperviously encapsulating an electronic component according to themethod of claim 1 wherein said electronic component is an integratedcircuit device which is assembled onto a flat prestamped lead framehaving plurality of leads which extend outside the area which isencapsulated.
 4. A method for imperviously encapsulating an electroniccomponent according to the method of claim 1 wherein during step (2) themold cavity is provided at a temperature of approximately 100° to 250°C., and the molding composition is introduced into the mold at atemperature of approximately 250° to 390° C. while under a pressure ofapproximately 100 to 1000 psi.
 5. A method for imperviouslyencapsulating an electronic component according to the method of claim 4wherein during step (2) the total molding cycle is conducted in lessthan one (1) minute.
 6. A method for imperviously encapsulating anelectronic component according to claim 1 wherein said moldingcomposition exhibits a melt viscosity within the range of approximately300 to 2500 poise at a shear rate of 100 sec.⁻¹ and at the temperatureit assumes while being injection molded.
 7. A method for imperviouslyencapsulating an electronic component according to the method of claim 1wherein said melt processable polymer which is capable of forming ananisotropic melt phase is selected from the group consisting of whollyaromatic polyesters, aromatic-aliphatic polyesters, wholly aromaticpoly(ester-amides), aromatic-aliphatic poly(esteramides), aromaticpolyazomethines, aromatic polyester-carbonates, and mixtures of theforegoing.
 8. A method for imperviously encapsulating an electroniccomponent according to the method of claim 1 wherein said meltprocessable polymer is wholly aromatic in the sense that each moietypresent contributes at least one aromatic ring.
 9. A method forimperviously encapsulating an electronic component according to themethod of claim 1 wherein said melt processable polymer is a whollyaromatic polyester.
 10. A method for imperviously encapsulating anelectronic component according to the method of claim 1 wherein saidmelt processable polymer is a wholly aromatic poly(esteramide).
 11. Amethod for imperviously encapsulating an electronic component accordingto the method of claim 1 wherein said melt processable polymer includesnot less than about 10 mole percent of recurring units which include anaphthalene moiety.
 12. A method for imperviously encapsulating anelectronic component according to the method of claim 1 wherein saidmelt processable polymer includes not less than about 10 mole percent ofrecurring units which include a naphthalene moiety selected from thegroup consisting of 6-oxy-2-naphthoyl moiety, 2,6-dioxynaphthalenemoiety, and 2,6-dicarboxynaphthalene moiety.
 13. A method forimperviously encapsulating an electronic component according to themethod of claim 1 wherein said melt processable polymer is a polyesterwhich may include amide linkages which was formed through apolymerization reaction in a polymerization zone of monomers to yield apolymer having recurring moieties selected from the group consisting ofthe following where in each instance Ar comprises at least one aromaticring: ##STR12## where Y is O, NH, or NR, and Z is NH or NR where R is analkyl group of 1 to 6 carbon atoms or an aryl group, ##STR13## where Zis NH or NR where R is an alkyl group of 1 to 6 carbon atoms or an arylgroup, and(f) mixtures of the foregoing;and wherein there was providedin the polymerization zone during said polymerization reaction anapproximately 1 to 4 percent molar excess of aromatic dicarboxylic acidmonomer and/or an esterified derivative thereof which during thepolymerization reaction imparted dicarboxyaryl units to the interior ofthe polymer chains of the resulting polymer and caused the polymerchains to terminate in carboxylic acid end groups and/or an esterifiedderivative thereof wherein the polymer chain achieved the requiredmolecular weight through the depletion of other monomers present in thepolymerization zone to yield a polyester product which was substantiallyincapable of additional chain growth upon subsequent heating asevidenced by an increase in the weight average molecular weight of nomore than 10 percent when heat in an inert atmosphere for 30 minuteswhile at a temperature of 340° C.
 14. A method for imperviouslyencapsulating an electronic component according to the method of claim13 wherein said polymerization reaction was carried out in the melt. 15.A method for imperviously encapsulating an electronic componentaccording to the method of claim 13 wherein any monomer present in thepolymerization zone which would otherwise include a hydroxyl groupand/or an amine group was provided as a lower acyl ester of about 2 to 4carbon atoms.
 16. A method for imperviously encapsulating an electroniccomponent according to the method of claim 13 wherein any monomerpresent in the reaction zone which would otherwise include a hydroxylgroup and/or an amine group was provided as an acetate ester.
 17. Amethod for imperviously encapsulating an electronic component accordingto the method of claim 13 wherein said polyester product exhibited aninherent viscosity of approximately 0.8 to 3.0 when dissolved in aconcentration of 0.1 percent by weight in pentafluorophenol at 60° C.prior being amixed with said particulate inorganic material.
 18. Amethod for imperviously encapsulating an electronic component accordingto the method of claim 13 wherein there was provided in thepolymerization zone during said polymerization reaction an approximately2.0 to 4.2 percent molar excess of aromatic dicarboxylic acid monomerand/or an esterified derivative thereof.
 19. A method for imperviouslyencapsulating an electronic component according to the method of claim13 wherein said melt processable polymer which is capable of forming ananisotropic melt phase has a weight average molecular weight ofapproximately 4,000 to 10,000.
 20. A method for imperviouslyencapsulating an electronic component according to the method of claim 1wherein said melt processable polymer is a wholly aromatic polyesterwhich was formed through a polymerization reaction in a polymerizationzone of ester-forming monomers to yield a polymer which consistedessentially of moieties I and II wherein:I is ##STR14## and II is##STR15## wherein said polyester comprised approximately 20 to 45 molepercent of moiety I, and approximately 55 to 80 mole percent of moietyII, and wherein there was provided in the polymerization zone duringsaid polymerization reaction an approximately 2.0 to 4.2 percent molarexcess of aromatic dicarboxylic acid monomer which during thepolymerization reaction imparted dicarboxyaryl units to the interior ofthe polymer chains of the resulting polymer and caused the polymerchains to terminate in carboxylic acid end groups wherein the polymerchains achieved the required molecular weight through the depletion ofother monomers present in the polymerization zone to yield a whoolyaromatic polyester product which was substantially incapable ofadditional chain growth upon subsequent heating as evidenced by anincrease in the weight average molecular weight of no more than 10percent when heated in an inert atmosphere for 30 minutes while at atemperature of 340° C.
 21. A method for imperviously encapsulating anelectronic component according to the method of claim 20 wherein saidmelt processable polymer which is capable of forming an anisotropic meltphase has a weight average molecular weight of approximately 4,000 to10,000.
 22. A method for imperviously encapsulating an electroniccomponent according to claim 1 wherein said melt processable polymer isa wholly aromatic poly(ester-amide) which was formed through apolymerization reaction in a polymerization zone of ester-forming andamide-forming reactants to yield a polymer which consisted essentiallyof moieties I, II, III, and IV, in the quantities indicated wherein ineach instance Ar is at least one aromatic ring, and wherein: ##STR16##II is ##STR17## III is

    --Y--Ar--Z--

where Y is O, NH, or NR, and z is NH or NR where R is an alkyl group of1 to 6 carbon atoms or an aryl group, and IV is

    --O--Ar--O--

wherein said poly(ester-amide) comprised approximately 40 to 80 molepercent of moiety I, approximately 5 to 30 mole percent of moiety II,approximately 5 to 30 mole percent of moiety III, and approximately 0 to25 mole percent of moiety IV; and wherein there was provided in thepolymerization zone during said polymerization reaction an approximately1 to 4 percent molar excess of aromatic dicarboxylic acid monomer whichduring the polymerization reaction imparted dicarboxyaryl units to theinterior of the chains of the resulting polymer and caused the polymerchains to terminate in carboxylic acid end groups wherein the polymerchains achieved the required molecular weight through the depletion ofother reactants present in the polymerization zone to yield a whollyaromatic poly(ester-amide) product which was substantially incapable ofadditional chain growth upon subsequent heating as evidenced by anincrease in the weight average molecular weight of no more than 10percent when heated in an inert atmosphere for 30 minutes while at atemperature of 340° C.
 23. A method for imperviously encapsulating anelectronic component according to the method of claim 22 wherein saidmelt processable polymer which is capable of forming an anisotropic meltphase has a weight average molecular weight of approximately 4,000 to10,000.
 24. A method for imperviously encapsulating an electroniccomponent according to claim 1 wherein said melt processable polymerwhich is capable of forming an anisotropic melt phase has a weightaverage molecular weight of approximately 4,000 to 10,000.
 25. A methodfor imperviously encapsulating an electronic component according toclaim 1 wherein said particulate inorganic material is present in saidmolding composition in a concentration of approximately 50 to 75 percentby weight based upon the total weight of the molding composition.
 26. Amethod for imperviously encapsulating an electronic component accordingto claim 1 wherein said particulate inorganic material has a weightaverage particle size of approximately 1 to 50 microns with at least 99percent by weight of the particles being below 100 microns, and anaverage aspect ratio of no more than 2.1.
 27. A method for imperviouslyencapsulating an electronic component according to claim 1 wherein saidparticulate inorganic material is particulate silicon dioxide.
 28. Amethod for imperviously encapsulating an electronic component accordingto claim 27 wherein said particulate silicon dioxide is fused silica.29. A method for imperviously encapsulating an electronic componentaccording to claim 1 wherein said particulate inorganic material isfused silica which bears a surface coating which aids in accomplishingits substantially uniform dispersal in component (a).
 30. A method forimperviously encapsulating an electronic component according to claim 1wherein said particulate inorganic material is fused silica which bearsa silane surface coating which aids in accomplishing its substantiallyuniform dispersal in component (a).
 31. A method for imperviouslyencapsulating an electronic component according to claim 1 wherein saidmolding composition includes less than 50 parts per million ofwater-extractable alkali metal, and less than 100 parts per million ofwater-extractable halogen.
 32. A method for imperviously encapsulatingan electronic component according to claim 1 wherein followingencapsulation the solidified molding composition exhibits a volumetriccoefficient of thermal expansion of no more than 150×10⁻⁶ cm. ³ /cm.³ °C. at 60° to 110° C.
 33. A method for imperviously encapsulating anelectronic component according to claim 1 wherein followingencapsulation the solidified molding composition exhibits a thermalconductivity of at least 10×10⁻⁴ cal.-cm./sec. cm.² ° C.
 34. A methodfor imperviously encapsulating an electronic component according toclaim 1 wherein following encapsulation the solidified moldingcomposition exhibits hydrolytic stability as evidenced by a retention ofat least 75 percent of the flexural strength thereof following 200 hoursin water at 110° C.